This invention relates to an electron tube, more particularly to an electron tube containing within its envelope a substance which improves the emission life of the cathode.
A typical electron tube such as a color cathode ray tube is usually provided with a front panel having phosphor screen on its inner surface, a funnel united with the panel and having conductive film on its inner surface, a neck united with the funnel and housing an electron gun, a shadow mask disposed in close proximity to the phosphor screen, and a magnetic inner shield which is assembled so as to be continuous with the shadow mask and extends along the inside face of the funnel. The phosphor screen comprises a phosphor layer consisting of at least phosphor dots or phosphor stripes emitting red, green and blue light and a metal backing layer formed on this layer. It is generally known also that to continuously maintain the degree of vacuum within the evacuated envelope, a metal getter film is formed on the inner surface of the funnel or the inner surface of the neck. The getter film absorbs the gases generated during operation of a color CRT from the various members described above which together constitute the color cathode ray tube, and thereby maintains the degree of vacuum. Generally speaking, when a color cathode ray tube is operating, there are produced a variety of gases, i.e., gas released from the vicinity of the cathode heater forming part of the electron gun, as a result of the heat from the heater; as released from the electrode members also forming part of the electron gun and from the shadow mask, due to the impinging on them of the electron beam emitted from the electron gun; and gas released as a result of the electron beam which has impinged on the shadow mask etc. being reflected and scattered and then re-impinging on the magnetic inner shield, inner conductive coating etc. Ionized by the beams of electrons which have been accelerated to a high voltage, these gases collide with the cathode surfaces of the abovementioned cathodes, and poison the electron emissive material of these cathode surfaces, thereby adversely affecting their emission characteristics. Further, when the temperature of the cathode surfaces etc. falls as the cathode ray tube is switched off, these gases which were produced during opertion are not only adsorbed on the getter film but are also adsorbed on the cathode surfaces, thereby poisoning the latter, and adversely affecting their emission characteristics. The principal constituents of the gases referred to above are H.sub.2 O, CH.sub.4 or the like. Water glass or sodium silicate is usually mixed with the graphite suspension mentioned earlier in order to strengthen the adhesion of the inner conductive film, of which the graphite suspension is the principal constituent, and this water glass, because of its great hygroscopicity, is a major source of gas production, which, as aforesaid, causes deterioration of the emission characteristics. The needs of construction of the neck diameter which is entailed by enlargement of the deflection angle of a color cathode ray tube, and the reduction in baking temperature in the exhaust process in order to shorten process time, make the deterioration in emission characteristics caused by the discharge gases mentioned above, and hence the reduction in emission life, still more marked. The emission life is a problem not only in color cathode ray tubes, but also in other electron tubes with a cathode, such as monochrome cathode ray tubes, travelling-wave tubes, magnetrons, klystrons, transmitting tubes and the like.
Finally, Japanese patent application Laid-Open No. 59-177833 discloses a technique for using SiO.sub.2 as a binder for the graphite conductive film, instead of the normally used water glass; but the function of the SiO.sub.2 here is that of a binder only, and it is not suggested that it improves emission.